Beam tube multiplexing system



BEAM TUBE MULTIPLEXHJG SYSTEM Rudolph A. Cola, Philadelphia, Pa., assignor to Burroughs Corporation, Detroit, Mich, a corporation of Michigan Application December 16, 1957, Serial No. 702,854

3 Claims. (Cl. 250-27) This invention relates to m ulti-position and mul-ti-output electron beam tubes, and particularly to animproved system for controlling and utilizing the output current in such tubes.

One type of multi-position tube to which the principles of the invention apply is shown in US. Patent No. 2,721,955 of Fan et al. Such a tube employs crossed electric and magnetic fields in its operation and includes a central longitudinal elongated cathode which is surrounded by ten groups of electrodes arrayed about the cathode. Each group of electrodes represents a position to which a beam of electrons may flow and each group includes a spade electrode adapted to form and hold an electron beam in the selected position and a target electrode which receives the electron beam and from which an output signal may be obtained. In addition, each group of electrodes may include a switching electrode which is employed to switch an electron beam from one position to the next. Generally, a tube of this type is a constant output current tube over a comparatively wide range of target electrode voltages, and up to the present time, a suitable system for modulating, or otherwise controlling, the output current from each target electrode has not been available.

Accordingly, one object of the present invention is to provide an improved system for operating a multi-position electron beam switching tube wherein the output current from each position may be separately controlled.

Another object of the invention is to provide a system for modulating the output current from each position of a multi-position electron beam switching tube.

Still another object of the invention is to provide an improved system for multiplexing a plurality of signals and obtaining a single serial output signal therefrom.

In brief, the principlesand objects of the invention are accomplished in a system which employs a multi-position electron beam tube which includes a central cathode and a plurality of groups of electrodes surrounding the cathode, each group including an electron beam forming and holding spade electrode and an output target electrode. A beam switching electrode may also be included in each group. A control or modulating signal is applied to one or more spade electrodes, and, as an electron beam is switched from position to position Within the tube, the signal coupled to each spade modulates the current flow to the target electrode associated with that spade. Each signal source is coupled to a spade electrode by means of a diode which controls the application of the signal voltage to the spade.

The output electrodes may be connected so that a separate modulated output current is obtained from each, or they may be connected together so that a single composite output signal is obtained. In addition, diflerent types of circuit elements, for example, tubes, transistors, magnetic cores, relays, or the like may be connected to each output electrode, and switched on or off in accordance with the control signal applied to the spade electrode.

2,919,347 Patented Dec. 29, 1959 The invention is described in greater detail with reference to the drawing wherein:

Fig. 1 is a perspective view, partly in section, of a type of electron beam switching tube employed in practicing the invention;

Fig. 2 is a schematic representation of the tube of Fig. 1 and a system in which it may be operated according to the invention;

Fig. 3 is a graph of characteristic operating curves of portions of the tube and circuit of Fig. 2; and

Fig. 4 is a schematic representation of a portion of a tube of the type shown in Fig. 1 and a modified circuit in which it may be operated.

The principles of the invention are particularly applicable to a multi-position, multiple-output electron beam switching tube of the type shown in U.S. Patent No. 2,721,955 to Fan et al. This type of tube 10 is shown in Fig. 1 and includes, briefly, an envelope 12 which contains a central longitudinally elongated cathode 14 and ten groups of electrodes, each spaced radially equidistantly from the cathode and surrounding the cathode. Each group of electrodes includes a generally U-shaped elongated spade electrode 16 and a target electrode 18 positioned so that each target occupies the space between adjacent spade electrodes. A generally rod-like switching electrode 29 is also included in each group of electrodes and is positioned between one edge of each target electrode and the adjacent spade electrode. An open-ended cylindrical permanent magnet 22 is provided surrounding the tube envelope and coaxial therewith. The magnet provides an axial magnetic field which is utilized in conjunction with electric fields within the tube to form and switch an electron beam from the cathode to each of the groups of electrodes.

Briefly, in operation of tube 10, electrons emitted by the cathode 14 are retained at the cathode if each of the spades, targets and switching electrodes carries its normal operating electrical potential. When a spade or switching electrode is suitably reduced in potential, an electron beam is formed and directed to an adjacent target electrode. The electron beam may be switched from one target electrode to the next by thus suitably altering the electrical potentials of a spade or switching electrode.

Referring to Fig. 2, there is shown a system embodying the principles of the invention and utilizing the tube 10. In the system shown in Fig. 2, the tube 10 is shown schematically in linear, rather than in circular, form and only six of the ten groups of spade, target, and switching electrodes are shown. The groups of electrodes are numbered 0, 1, 2, 3, 8, 9 to identify the various positions at which an electron beam may be formed in the tube. The cathode 14 of the tube is connected to ground. The switching grid electrodes 20 may be connected in several difierent ways for performing the desired switching of an electron beam from one group of electrodes, or positions, to the next. In one suitable arrangement, the even-numbered switching electrodes are connected together in a first set, and the odd-numbered electrodes are connected together in a second set, and the two sets of switching electrodes thus formed are connected to the two output terminals of a suitable flip-flop circuit. Each target electrode 18 is connected through a load resistor 28 to a common target buss 30 which is coupled through a common target load resistor 32 to a source of positive supply voltage of about volts. If desired, a separate output lead (not shown) may be connected to each target electrode. The spade electrodes 16 are connected through spade resistors 34 to a spade buss 36 which is coupled, in turn, to the 100 volt positive voltage supply source.

According to the invention, each spade electrode 16 is connected through a unidirectional current flow device 38 to a signal source 40. The unidirectional device is provided to isolate each signal source from its associated spade electrode, and may comprise a diode which has its cathode connected directly to the spade and its anode connected to the output of the signal source. The signal source 40 is also connected to ground in conventional fashion. The signal source may be biased at any suitable potential, and in the circuit of Fig. 2 it is biased at zero volts.

For a description of the operation of the invention, reference is made to Fig. 3 which shows the current-voltage steady-state characteristic curve, j, for any one of the spade electrodes. The corresponding target current, I curve for the target associated with a spade is also shown. The load line for a spade series resistor R, intersects the j curve at three points: (1) point Z at which the spade is electrically at its supply voltage and carries zero current, which means that the electron beam in the tube is cut off; (2) point X which is on the portion of the j characteristic curve having a negative-resistance slope and thus is an unstable position; and (3) point Y which is on the portion of the j curve having a positive-resistance slope and represents a stable position at which an electron beam is turned on and flowing to a target. Ordinarily, a spades voltage and current stabilize at point Y.

in the system of Fig. 2, the beam tube it! can perform its normal counting or switching function independently of the signal sources 4% and whether or not signals are generated thereby. When the sources 4% do generate signals, the operation of the system is as follows. Jhen the tube 10 has been cleared and no electron beam is present in any of the positions, each of the spade electrodes 16 carries a potential of about 100 volts and each diode 38 is biased in the reverse direction. Current from the signal sources 4'8 is thus prevented from reaching the spade electrodes. This condition of the spade electrodes is represented by point Z in Fig. 3. When an electron beam forms in one of the tube positions, for example, the position, the voltage of the spade electrode at the 0 position is lowered until it reaches zero volts or becomes slightly negative. At the same time, the cathode of the diode 38 at the 0 position is reduced to zero volts or to a slightly negative voltage, and the diode is biased in the forward direction. The diode then conducts current and clamps the spade electrode to the bias voltage which is zero volts in this case. At this time, signal current from the Source 40 at the 0 position can pass through the diode to the G spade and modulate the beam current to the target 13 at the 0 position. If, for example, a sinusoidal signal voltage 41 (Fig. 3) is generated by the source 40, the variations of this signal voltage are reflected in variations along the spade load line between the points A and B and along the j characteristic curve between the points A and B'. At the same time, referring to the target current curve, I it can be seen that the target output current varies between points A" and B" and an output current wave 43 is provided. Thus, the target or output current from each of the positions in the tube is independently modulated by a signal from the corresponding signal source '40.

The curves of Fig. 3 show that, at any instant, the beam current divides into two portions; one portion passing into the spade resistor, I is represented by the height of the line between the horizontal aXis and the spade load line, and the other portion passing into the diode, I is represented by the height of the line between the spade load line and the spade characteristic curve i.

In using the system of Fig. l, the modulating signal from each source 40 may have substantially any desired Wave shape, and it may have a maximum voltage amplitude swing which extends between the points X and Y on tne spade load line. In addition, if desired, clipping of an input signal may be achieved if the signal voltage extends beyond the point Y in the negative voltage direction. In

this region, the diode 35 is biased in the reverse direction, and no signal reaches the spade electrode. The amplitude Swing of the signal should not be so great that the point X is exceeded or the electron beam in the tube will clear; that is, electrons will remain in the vicinity of the cathode and will not form at any position because of the negative resistance slope of the spade characteristic curve. When the tube is cleared, all spades are at the potential represented by the point Z.

The circuit of Fig. 2 may be adapted to perform a control or switching operation for many different types .of utilization circuits which may be coupled to the target electrodes of a multi-position electron beam tube such as the tube lll One example of such a circuit is shown in Fig. 4. Only two positions or groups of electrodes of a multi-position electron beam tube are shown, each including a spade electrode 50, a target electrode 52, and a switching electrode 54. Each spade electrode 50 is connected through a diode 56 to a signal source 58 and then to ground for zero bias. Each target electrode 52 is coupled to a circuit including a PNP transistor 60, the operation of which is controlled thereby. Each target is also connected through a load resistor 62 to the junction point 64 of two resistors 66 and 63, one of which, 66, is connected to ground, and the other of which, 68, is connected to a source of positive supply voltage. The junction point 64 of the two resistors is connected to the base electrode 79 of the transistor which has its emitter electrode '72 connected to ground, and its collector electrode 74 connected through a load resistor 76 to a suitable source of negative bias voltage V The same type of transistor circuit may be connected to each target electrode of the beam tube.

According to the invention, the circuit components and supply voltages coupled to the transistor 60 are selected so that, normally, without a signal being generated by any of the signal sources 58, as an electron beam in the tube moves from position to position, the target output current is too small to bias the base electrode of the transistor negative with respect to the emitter electrode and thus cause the transistor to conduct current. At any instant, however, when the beam is at a selected spade electrode and target electrode and it is desired to switch on the'transistor connected thereto, the appropriate signal source 58 is turned on and the signal generated thereby, for example a negative pulse 78, is applied to the appropriate spade electrode 50. The applied signal voltage increases the target output current flow through the load resistor 62 and, as a result, the transistor base electrode is now biased negative with respect to the emitter electrode. The transistor is thus switched to its conductive state and an ouput signal may be obtained therefrom.

The principles illustrated in Fig. 4 may also be employed to operate other devices such as magnetic cores, relays, or the like which are coupled to the target electrodes of a multi-position beam tube.

In any of the circuits described above, modifications may be made within the scope of the invention. For example, the specific type of multi-position tube described above need not be employed. In addition, the signal generators may be either D.C. coupled, as shown, or AC. coupled through capacitors to the current-controlling electrodes of the tube.

A system embodying the principles of the invention has many advantages over comparable systems in the prior art. Perhaps most important is the fact that the invention provides a simple and comparatively inexpensive system for multiplexing a plurality of signals and for controlling a plurality of circuits with said signals. In addition, in the operation of the invention, the output voltage in each target load resistor is in phase with the input signal voltage. Also, as shown in the curves of Fig. 3, the output current is linearly related to the input signal, since the target current curve is linear over most of its range.

What is claimed is:

1. A multiplexing and modulating system including an electron discharge device adapted to be operated with crossed electric and magnetic fields and including a central axially elongated cathode and a plurality of groups of electrodes surrounding said cathode; each of said groups of electrodes including a current receiving target electrode, a spade electrode adapted to form and hold an electron beam current on an associated target electrode, and an electron beam switching electrode; and a separate signal source coupled through a diode to each spade electrode for applying a signal to each of said spade electrodes.

2. A multiplexing and modulating system including an electron discharge device adapted to be operated with crossed electric and magnetic fields and including a central cathode, a plurality of output electrodes surrounding said cathode and being adapted for separately receiving current from said source, and a plurality of spade electrodes surrounding said cathode and being adapted to form and hold an electric current on an associated output electrode; and separate signal sources each coupled through a diode to each of said spade electrodes.

3. A multiplexing and modulating system including an electron discharge device adapted to be operated with crossed electric and magnetic fields and including a source of electric current, a plurality of output electrodes, each of which is adapted for separately receiving current from said source, and a plurality of spade electrodes adapted to form and hold an electric current on an associated output electrode; a separate signal source coupled through a diode to each of a plurality of said spade electrodes; and a circuit coupled to at least one of said target electrodes, said circuit including a circuit element having two operative states, said circuit element being adjusted so that it is in one state when no signal is applied to a spade electrode and it is in its other state when a signal is applied to a spade electrode from a signal source coupled thereto.

References Cited in the file of this patent UNITED STATES PATENTS 2,591,997 Backmark Apr. 8, 1952 2,659,814 Sternbeck Nov. 17, 1953 2,721,955 Fan et al. Oct, 25, 1955 2,731,571 Chance Jan. 17, 1956 

